In response to the demands of consumers who are driven both by ever-escalating fuel prices and the dire consequences of global warming, the automobile industry is starting to embrace the need for ultra-low emission, high efficiency cars. While some within the industry are attempting to achieve these goals by engineering more efficient internal combustion engines, others are incorporating hybrid or all-electric drive trains into their vehicle line-ups.
In recent years electric vehicles (EVs) have proven to be not only environmentally friendly, but also capable of meeting, if not exceeding, consumer desires and expectations regarding performance, range, reliability, and cost. In order to insure both vehicle reliability and user safety, electric vehicles use a variety of techniques to prevent battery pack abuse as well as mitigate the effects of an unavoidable abusive event (e.g., battery pack damaged during a collision, etc.). Fuses, which may be employed at the battery level, the battery pack level, or both, are one of the primary means of protecting an EV's battery pack. Unfortunately while fuses may be used to provide very effective protection in a low current circuit, due to the high current levels common in an EV the response time of a fuse may be too slow to provide the desired level of protection. This phenomenon is illustrated in FIG. 1 which provides the cutoff current characteristics for a variety of conventional high current fuses ranging from a 300 amp fuse to an 800 amp fuse. As expected, as the current rating of the fuse increases, so does the time it takes to blow the fuse for a given current level. Thus for the set of exemplary fuses shown in FIG. 1, a 300 amp fuse subjected to 1000 amps of current will take approximately 8 seconds to blow while a 600 amp fuse may take as much as 200 seconds to blow at the same current level. Subjecting an EV's electrical system to an overcurrent of such magnitude and for such an extended period of time may damage the battery pack. To avoid this problem, the fuse within an EV's power train is typically undersized to insure that the fuse will blow quick enough to protect the various battery pack and drive train components. For example, assuming that the EV battery pack uses wire bond battery interconnects that typically are only capable of withstanding 1000 amps for approximately 10 seconds, based on the above fuse data a 300 amp fuse would be required to insure adequate protection.
Unfortunately while undersizing the fuse may provide the desired level of protection for the battery pack, under certain routine conditions the fuse may blow prematurely. In part this is due to the thermal characteristics of the wire bond versus those of the fuse. FIG. 2 graphically illustrates the heat-up and cool-down cycling of a wire bond interconnect (curve 201) versus that of a 300 amp fuse (curve 203) as the system is subjected to a series of aggressive current pulses as illustrated in FIG. 3. Such a pulse pattern may be due, for example, from an aggressive driving pattern such as those that may occur during street racing or otherwise spirited driving. As shown, eventually the fuse becomes too hot, resulting in the system going into an overheat protection condition, i.e., the fuse blows prematurely.
Accordingly, what is needed is a fuse that provides a rapid response to excessive currents while still insuring that the fuse will not blow during normal vehicle operation. The present invention provides such a system.